This invention relates to optically readable information discs (or simply optical discs) such as digital audio discs, optical video discs, memory discs, etc., read out by light.
Optical discs are formed, for example as shown in FIG. 1, by placing oppositely two optical disc base plates comprising a transparent substrate 1 such as a glass plate, a transparent plastic plate (made from poly(methyl methacrylate), polycarbonate, etc.), a primer layer 2 formed on the substrate 1 and having groove-like and hole-like depressions and protuberances for servo tracks, and a metal-made recording film 3 formed on the primer layer 2 using such a metal as Bi, In, Te, Se, As, Pb, Sn or the like, so as to face the recording films 3 each other using a spacer 4 with a predetermined distance, and bonding the spacer and the transparent substrates with an adhesive (not shown in the drawing). Then, the recording film 3 is irradiated with a light beam and the irradiated portion of the metal-made recording film is removed to give information units.
Such optical discs are produced as shown in FIGS. 2(a) to 2(c). That is, an ultraviolet-curable resin composition is inserted between a stamper 5 made of a metal such as nickel and having predetermined depressions and protuberances, and a transparent substrate 1, followed by curing the resin composition with ultraviolet light to form a primer layer 2 [FIG. 2(a)]. Then, the metal-made stamper 5 is taken off [FIG. 2(b)]. A metal-made recording film 3 is formed on the primer layer having depressions and protuberances [FIG. 2(c)], and two thus formed optical disc base plates (comprising the transparent substrate, the primer layer and the recording film) are placed oppositely so as to face the metal-made recording films each other using spacers with a predetermined distance, followed by bonding the spacers to the optical disc base plates.
The ultraviolet-curable resin composition used for the primer layer mentioned above should satisfy the following conditions:
(a) The cured product is optically transparent. PA0 (b) The resins adheres well to the transparent substrate made from glass, a transparent plastic, or the like. PA0 (c) The resin is easily peeled off from the stamper. PA0 (d) The metal-made recording film is rapidly melted and removed by the irradiation with a light beam to easily give round good holes. PA0 (e) The metal-made recording film is not deformed due to moisture absorption when used for a long period of time, nor generates cracks due to the swelling of the recording film. Further, it should be prevented that a corrosive substance realeased from a cured product due to moisture absorption oxidizes the metal-made recording film to increase the transmittance of the recording film. That is, recorded signals should be read out correctly even under moisture absorbed circumstances. PA0 (f) Thermal stability of grooves with depressions and protuberances on the metal-made recording film obtained by vacuum deposition is excellent without causing changes in grooves' height and shape and without generating cracks. PA0 (g) To have mechanical strength enough to withstand a high-speed revolution of 50 rpm or more is necessary. PA0 (a) 20 to 80 parts by weight of a compound of the formula: ##STR1## wherein R.sub.1 is hydrogen or a methyl group; R.sub.2 and R.sub.3 are independently hydrogen or an alkyl group having 1 to 8 carbon atoms; and R.sub.4 is ##STR2## (b) 80 to 20 parts by weight of a compound of the formula: ##STR3## wherein R.sub.1 and R.sub.5 are independently hydrogen or a methyl group; and n is an integer of 5 to 12, and PA0 (c) a photopolymerization initiator in an amount of 0.1 to 5.0 parts by weight per 100 parts by weight of the total of the components (a) and (b).
But conventional ultraviolet-curable resin compsoitions were not able to satisfy the above-mentioned conditions (a) to (g) at the same time.
A typical example is shown in Japanese Patent Unexamined Publication No. 127946/81.